I used to be with EE and leaving Paddington there was always a drop out zone in the Ladbroke Grove area. I remember a friend on another network telling me he had the same issue there.
If you look on
Opensignal that gap shows up clearly - Golborne Road is a mass of red. At Paddington, the only red patch is for 3 (3G and 4G) centred just outside the station at the "front" end. The other networks look fine. In this user-rich environment Opensignal should be pretty reliable, too.
But that's about signal levels (I don't think they even collect speed data), and the
OP▸ said that wasn't his issue. If you get signal but no traffic, that must be down to the balance of base station channels and backhaul capacity versus user load - so who knows?